In recent years, along with strengthened environmental regulations concerning fluorine, there is an increased need for a method for removing fluorine from waste liquid generated in manufacturing process of fluorine compound such as hydrochloric acid and sulfuric acid containing fluorine.
Conventionally, as a method for removing fluorine in an aqueous solution, a method of adding calcium to a fluorine-containing aqueous solution and adjusting pH of the aqueous solution to neutral to precipitate calcium fluoride is known (Non Patent Literature 1). However, when fluorine in an acidic aqueous solution is removed by this method, for example, the aqueous solution after removal of fluorine becomes neutral. Therefore, it is impossible to reuse the acidic aqueous solution after removal of fluorine.
On the other hand, Patent Literature 1 describes a method for purifying hydrochloric acid comprising: a step for bringing a silicon compound such as trimethylchlorosilane into contact with the hydrochloric acid containing hydrogen fluoride; and a step for recovering trialkylfluorosilane compound formed in the contacting step. In this method, the recovery step consists of a step for hydrolyzing the trialkylfluorosilane compound to convert it into a trialkylsilanol compound and a step for condensing the trialkylsilanol compound to convert it into a hexaalkyl disiloxane compound. The hexaalkyl disiloxane compound obtained in the recovery step is chlorinated to form trialkylchlorosilane compound, and the trialkylchlorosilane compound is reused in the contacting step. Hydrochloric acid having higher concentration of more than 25% by weight is required to chlorinate the hexaalkyl disiloxane compound.
Patent Literature 2 describes a method for processing a fluorine-containing waste liquid, characterized by filling the fluorine-containing waste liquid with a solid adsorbent on which hexaalkyl disiloxane is supported, blowing an air into the waste liquid while maintaining the waste liquid in an acidic state, guiding a fluorotrialkylsilane scattered from the waste liquid into an absorption vessel receiving an alkali solution and a solid adsorbent, forming a trialkylsilanol and a fluorine ion in the absorption vessel, and recovering the fluorine ion in the alkali solution while adsorbing the trialkylsilanol as a hexaalkyl disiloxane on the solid adsorbent. This method requires as long as three hours to remove fluorine from the fluorine-containing waste liquid. Further, since this method requires the handling of the solid adsorbent and fluorotrialkylsilane scattered with the air in addition to the fluorine-containing waste liquid and hexaalkyl disiloxane which are liquid, the processing in this method is cumbersome and can bring about high cost.